Chemical changes to electrodes in a rechargeable battery may cause degradation in the battery's capacity, duration of charge retention, charging time, and other functional parameters. Battery degradation may accumulate over the life of the battery. Environmental factors (e.g., high temperature) and functional factors (e.g., improper charging and discharging) may accelerate battery degradation. Operators of systems that rely on rechargeable battery power may desire to monitor the degradation of the batteries they use. One indicator of battery degradation is an increase in battery impedance.
FIG. 1 is an impedance plot 102 of a fresh battery and an impedance plot 104 of an aged battery, measured at several different frequencies using an Electrochemical Impedance Measurement (EIM) system. The Y-axis is the imaginary impedance and the X-axis is the real impedance for a plurality of different frequencies plotted in FIG. 1. As illustrated in FIG. 1, the aged battery (plot 104) exhibits a higher impedance than the fresh battery (plot 102) at each of the different frequencies. Operators of systems that rely on rechargeable batteries may use impedance data, such as the impedance data of FIG. 1, to determine that a replacement battery is needed before a failure occurs. Such preemptive replacement may prevent expensive delays and property damage that may occur in the event of a battery failure. Also, knowledge of a battery's continued reliability may prevent expenses associated with unnecessarily replacing a battery that still has a substantial amount of lifetime left.
Existing impedance measurement systems have a resolution of approximately 0.1 mOhm when operating at an excitation current range of approximately 500 mA. As a result, existing impedance measurement systems may be able to determine the impedance of a test battery exhibiting a 10 mOhm internal impedance with adequate resolution. The resolution of existing impedance measurement systems may limit the ability to test batteries exhibiting lower internal impedance (e.g., 1 mOhm). Other methods of impedance measurement (e.g., electrochemical impedance spectroscopy) may achieve high resolution, but may be slow to be tuned, such as requiring time on the order of about 10 minutes to obtain the measurements.